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Glass Thermal Stress and Cracking

Resistance of glass to thermal stress and cracking

Glass is a very fragile product and cracks very easily when stressed.

When glass is exposed to solar radiation, it absorbs heat, rises in temperature and expands. The edges of the glass in it's frame which are not exposed to the radiation remain cooler than the exposed area.

The force generated by the expansion from the heat of the sunlight in the direct exposed glass area causes stress, which can create the glass to crack. This is known as a thermal fracture.

Although thermal breakages are more often associated with solar radiation, a similar effect can be obtained in darkness. Thermal cracks can also occur in cold climate if there has been a rapid decrease in the outside temperature. So whenever we have rapid changes in temperatures glass is at risk of thermal cracking. This is one of the reasons glazers are in business replacing cracked glass.

Hot centres and cold edges create stress. Glass can also crack when putty gets hard after years and the glass can no longer expand when the glass heats up. Stress is generated by the large area of hotter glass expanse causing a much smaller cold area to stress.

Consideration needs to be taken when installing window film onto laminated glass or even very large panes of float glass. Dark high absorbing window film can cause some cracking and precaution needs to be taken. Advice needs to be obtained from professional film consultants on which products not to use on laminated glass.

Silver 20 is the only highly reflective film we recommend for laminated glass as it has a very low absorption factor and reduces the risk of glass cracking.

Another reason glass cracks is because the glazing was not done properly and under the beading the edges are rough and in some cases touching the edge of the frames. When glass is installed the correct way is to sand the edges smooth and make sure there is room for expansion. Rough edges are risky and most times when glass breaks and is replaced one can see the rough edges after removing the glass and putty. The larger and thicker the pane is, the more difficult it is to cut, handle and glaze without causing damage to the edges and introducing potential sources of cracking at a later stage.

Glass is also known to crack when one has a dark blind or curtain at the back of it. This interferes with ventilation and free movement of air around the glass. The black surface allows for absorption of the heat between the glass and the blind, causing a high heat build up which is absorbed by the glass resulting in cracking.

Glazing contractors hide their poor workmanship under the edge of the beading because they know nobody will see it. Once it cracks and glass is removed the bad workmanship raises it’s head. Therefore the condition of the edge of a pane of glass is extremely important.

Toughened glass will never be subject to stress as it goes through a heating and cooling process. The outdoor air temperature may vary considerably from a minimum just before dawn to a maximum around mid afternoon. This can also cause glass to stress.

Sometimes there is an overhang on a building which causes a shadow across the glass and this situation can also cause stressing of glass as the one half gets hot and the shaded area remains relatively cool. There is a difference in expansion stressing the glass.


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